It is common knowledge to sense the presence of an object using capacitive sensing techniques. Illustratively, an oscillator is utilized to deliver a field generating signal to a sensing element wherein the sensing element operates as one of the plates of a capacitor that produces an electric field.
When a conductive or grounded object enters the electric field, it serves as the second plate of the capacitor whereby the capacitance formed between the sensing element and the object changes as the distance varies between the object and the sensing element.
When such techniques are used in conjunction with a parking gate, an error signal is sent to a control unit to stop the movement of a gate that is being lowered upon an approaching vehicle. This technology is disclosed in international application PCT/US01/4253.
A disadvantage associated with the above capacitive sensing technique is that it adapts itself too quickly to a changing environment, that is, when used as a security system it may allow a person to move very slowly towards a protected object without being detected. More particularly, the system includes a phase lock loop (PLL) which adjusts the reference frequency signal in relation to the sensing field signal in a very short time such that the slowly approaching object may not be detected.
Accordingly, there exists a need for a capacitive sensing system that measures frequency shift in response to an approaching object which does not have the disadvantages of a phase lock loop system thereby providing a more reliable and robust system for use in the security industry as well as other applications.